The IEEE 802.16 Working Group (WG) is established two standards, one of which, 802.16d, is primarily for using in fixed-line communications, while the other, 802.16e, is for using in mobile communications (see for example IEEE Std. 802.16™-2004 and IEEE Std. 802.16e™-2005).
FIG. 10 shows an example of the frame configuration in 802.16d/e. A base station generates such a frame and transmits the frame to a mobile station. The frame DL/UL-MAP includes DL/UL subframe configuration information and communication control information. By referencing DL/UL-MAP, the mobile station performs DL-direction reception processing and UL-direction transmission processing.
FIG. 11A and FIG. 11B show examples of allocation of physical subchannel to UL subframe in the 802.16e standard. In 802.16e, physical subchannel allocated to the UL subframe is specified in bitmap format (“0” or “1”). In the example of FIG. 11A, physical subchannels #1, #2, . . . , #N−1 are allocated to the UL subframe, and physical subchannels #0, #3, and similar are not used.
Also, in 802.16e, as the method of allocation of physical subcarriers (hereafter “subcarriers”) forming physical subchannels, band AMC (Adaptive Modulation Coding) and PUSC (Partial Usage of SubChannels) may be used. In band AMC, adjacent subcarriers on frequencies are mapped to physical subchannels. On the other hand, in PUSC, distributed subcarriers on different frequencies are mapped.
FIG. 12 shows an example of PUSC mapping of physical subchannels and subcarriers. 4 subcarriers×3 OFDMA symbols are taken to be one tile, and one physical subchannel is formed from six distributed tiles. The six tiles forming the physical subchannel may for example be selected using the following formula.Tiles(s,n)=Nsubchannels*n+(Pt[(s+n)mod Nsubchannels]+UL_Permbase) mod Nsubchannels  (1)
In equation (1), s is a number of the physical subchannel, n is a tile index value, Nsubchannels is a total number of physical subchannels (when FFT=1024, 35 physical subchannels), Pt[ ] is a permutation matrix, and UL_Permbase is a permutation seed value set by the base station.
By generating such distributed physical subchannels, subcarriers are distributed within UL subframes, and averaged values are assumed for UL subframes as a whole, so that wireless communication quality is improved.
Thereafter, the base station maps the generated physical subchannel to the logical subchannels of UL subframes. With respect to this mapping, the 802.16e standard stipulates application of data subchannel rotation (hereinafter referred to as “subchannel rotation”) in PUSC.
FIG. 13 shows an example of mapping when subchannel rotation is applied. Subchannel rotation is a mapping method in which, when a slot (in UL PUSC, one subchannel×3 OFDMA symbols) changes (changes to the next time slot), the mobile station modifies the mapping of physical subchannels and logical subchannels. In the example in the figure, physical subchannels #1, #2, #5, #11 are mapped in order in the time axis direction to the logical subchannel #0, and physical subchannels #2, #5, #11, #17 are mapped to logical subchannel #1. Focusing on physical subchannel #2, in the first slot time physical subchannel #2 is mapped to logical subchannel #1, in the next slot time mapped to logical subchannel #0, and in the next slot time mapped to logical subchannel #M−1. By means of such mapping, the mobile station can use different frequencies in each slot, and immunity to fading is enhanced.
Studies on an 802.16m specification, as a next version of 802.16e, are currently beginning (see for example http://www.ieee802.org/16/tgm/contrib/C80216m-08—063r1.pdf). FIG. 14 shows an example of a frame configuration in 802.16m. Studies are in progress to enable the 802.16m specification to support not only mobile stations conforming to 802.16m, but also mobile stations conforming to 802.16e. As shown in the figure, the UL subframe is divided into two frequency regions, one of which is a transmission region for mobile stations conforming to 802.16e, while the other is a transmission region for mobile stations conforming to 802.16m.
Studies of 802.16m are underway to make an OFDM symbol CP (Cyclic Prefix) region in the UL subframe smaller than in 802.16e. FIGS. 15A to 15C illustrate an example of the relations between OFDMA symbols and UL subframes. 802.16m can reduce the CP region, so that the number of OFDMA symbols arranged in one frame can be increased, and the volume of data which can be transmitted can be increased.
On the other hand, in the WiMAX Forum, FFR (Fractional Frequency Reuse) is proposed as the mode of utilization of frequency bands based on 802.16d/e (see for example “Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation” (August 2006)). FIG. 16 shows an example of an UL subframe configuration in FFR. In FFR, for example, the UL subframe is divided in two regions in the frequency axis direction, with one a frequency region which is different from that of adjacent base stations (R3 region), and the other a frequency region which is the same for adjacent base stations (R1 region). The mobile station at a cell edge is allocated to frequency region in the R3 region, and mobile station near base stations is allocated to frequency region in the R1 region. By this means, efficiency of frequency utilization is improved and coverage is expanded.
However, if the PUSC (FIGS. 12A to 12D) and subchannel rotation (FIG. 13) stipulated in 802.16e are applied to the UL subframes of the above-described 802.16m (FIG. 14), due to shifting of the starting position of time-axis slots, in some cases there occur time bands in which the same frequency is used by different mobile stations subordinate to the same base station at the same time.
FIG. 17 is used to explain this situation. As shown in the figure, when the mobile station conforming to 802.16e uses the frequency band of physical subchannel #1 in the initial slot time, there is a time band in which the mobile station conforming to 802.16m uses the frequency band of the same physical subchannel #1. In such a case, two different mobile stations are using the same subcarrier in the same time band, and physical subchannels collide. Due to such physical subchannel collision, deterioration of the base station reception characteristics and other problems with deterioration of wireless communication quality occur.
Further, when PUSC and subchannel rotation are applied to FFR UL subframe, a similar problem of collision of physical subchannels used by adjacent base stations occurs. This is because subchannel rotation is performed for UL subframe overall by the base station and by the adjacent base station, so that there are cases in which the same physical subchannels are mapped in the same time bands in each of the R3 regions of the base station and adjacent base station. Due to physical subchannel collision, a portion of the R3 region allocated to the mobile station on the cell edge is used by a mobile station of the adjacent base station in the same time band, so that the base station receives interference from the mobile station of the adjacent base station. In this case also, physical subchannel collision causes deterioration of wireless communication quality.
In other wireless communication methods also, similar problems can occur when physical subchannel modifications are performed.